Variation in stickleback head morphology associated with parasite infection NIELS J. DINGEMANSE 1 *, CHRIS OOSTERHOF 1 , FONS VAN DER PLAS 1 and IAIN BARBER 2 † 1 Animal Ecology Group, Centre for Ecological and Evolutionary Studies and Department of Behavioural Biology, Centre for Behaviour and Neurosciences, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands 2 Institute of Biological Sciences, University of Wales Aberystwyth, Aberystwyth SY23 3DA, Wales, UK Received 7 January 2008; accepted for publication 2 September 2008 Parasites can affect host phenotypes, influencing their ecology and evolution. Host morphological changes occurring post-infection might result from pathological by-products of infection, or represent adaptations of hosts or parasites. We investigated the morphology of three-spined sticklebacks, Gasterosteus aculeatus, from a population naturally infected with Schistocephalus solidus, which grows to large sizes in their body cavity. We examined local effects of infection on trunk shape, which are imposed directly by the bulk of the growing parasite, and distant effects on head morphology. We show that trunk shape differed between infection classes, and was affected more severely in fish with heavier total parasite mass. We further show unexpected differences in head morphology. The heads of infected fish were reduced in size and differently shaped to those of non-infected fish, with infected fish having deeper heads. Importantly, both head size and shape were also affected more severely in fish with heavier total parasite mass. This latter result suggests that differences in morphology are caused by post-infection changes. Such changes may be incidental, evolutionarily neutral ‘side effects’ of infection. However, because head morphol- ogy affects foraging ecology, such changes are likely to have fitness consequences for hosts, and may constitute adaptations, either of hosts or of parasites. We discuss our finding in the context of the evolution of phenotypic plasticity, and suggest testable hypotheses examining the proximate mechanisms underlying these morphological effects and their potential evolutionary basis. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 759–768. ADDITIONAL KEYWORDS: body shape – feeding ecology – head shape – host – parasite evolution – phenotypic plasticity – Schistocephalus solidus. INTRODUCTION Parasite infections can impact a wide range of host phenotypic characteristics, including morphology, physiology, and behaviour (Poulin & Thomas, 1999; Moore, 2002). Parasites therefore have profound implications for the ecological interactions of hosts (Lafferty & Morris, 1996) and can have long-term evolutionary consequences for host populations (Laf- ferty, 1999; Poulin & Thomas, 1999). Most intuitively, parasites can drive the evolution of parasite avoid- ance and reduction behaviours, including immune systems (Hart, 1990; Wakelin & Apanius, 1997). Fur- thermore, if parasites are common and negatively impact the survival and/or reproduction of hosts, they may also influence the evolution of life history traits in host populations (Lafferty, 1993; Fredensborg & Poulin, 2006), much as predators do (Reznick & Endler, 1982). Parasitism may also play a role in the evolution or maintenance of phenotypic plasticity in hosts. Infec- tions are often associated with changes in host behav- iour, including habitat use (Barber, Hoare & Krause, 2000; Moore, 2002), expanding the range of habitats *Corresponding author. E-mail: n.j.dingemanse@rug.nl †Current address: Department of Biology, University of Leicester, Leicester LE1 7RH, UK. Biological Journal of the Linnean Society, 2009, 96, 759–768. With 4 figures © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 759–768 759